The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Emissions control is an important factor in engine design and engine control. Oxides of nitrogen, NOx, are known by-products of combustion. NOx is created by nitrogen and oxygen molecules present in engine intake air disassociating in the high temperatures of combustion, and rates of NOx creation include known relationships to the combustion process, for example, with higher rates of NOx creation being associated with higher combustion temperatures and longer exposure of air molecules to the higher temperatures. Reduction of NOx created in the combustion process and management of NOx in an exhaust aftertreatment system are priorities in vehicle design.
NOx molecules, once created in the combustion chamber, can be converted back into nitrogen and oxygen molecules in exemplary devices known in the art within the broader category of aftertreatment devices. However, one having ordinary skill in the art will appreciate that aftertreatment devices are largely dependent upon operating conditions, such as device operating temperature driven by exhaust gas flow temperatures and engine air/fuel ratio. Additionally, aftertreatment devices include materials, such as catalyst beds, prone to damage or degradation as a result of use over time and exposure to high temperatures.
Modern engine control methods utilize diverse operating strategies to optimize combustion. Some operating strategies, optimizing combustion in terms of fuel efficiency, include lean, localized, or stratified combustion within the combustion chamber in order to reduce the fuel charge necessary to achieve the work output required of the cylinder and increase engine efficiency, for example, by operating in an unthrottled condition, reducing air intake pumping losses. While temperatures in the combustion chamber can get high enough in pockets of combustion to create significant quantities of NOx, the overall energy output of the combustion chamber, in particular, the heat energy expelled from the engine through the exhaust gas flow, can be greatly reduced from normal values. Such conditions can be challenging to exhaust aftertreatment strategies, since, as aforementioned, aftertreatment devices frequently require an elevated operating temperature, driven by the exhaust gas flow temperature, to operate adequately to treat NOx emissions.
Aftertreatment devices are known, for instance, utilizing chemical reactions to treat exhaust gas flow. One exemplary device includes a selective catalytic reduction device (SCR). Known uses of an SCR device utilize ammonia derived from urea injection to treat NOx. Ammonia stored on a catalyst bed within the SCR reacts with NOx, preferably in a desired proportion of NO and NO2, and produces favorable reactions to treat the NOx. One exemplary embodiment includes a preferred one to one, NO to NO2 proportion, and is known as a fast SCR reaction. It is known to operate a diesel oxidation catalyst (DOC) upstream of the SCR in diesel applications to convert NO into NO2 for preferable treatment in the SCR. Continued improvement in exhaust aftertreatment requires accurate information regarding NOx emissions in the exhaust gas flow in order to achieve effective NOx reduction, such as dosing proper amount of urea based on monitored NOx emissions.
Other aftertreatment devices are additionally known for treating the exhaust gas flow. Three way catalysts (TWC) are utilized particularly in gasoline applications. Lean NOx traps (NOx trap) utilize catalysts capable of storing some amount of NOx, and engine control technologies have been developed to combine these NOx traps or NOx adsorbers with fuel efficient engine control strategies to improve fuel efficiency and still achieve acceptable levels of NOx emissions. One exemplary strategy includes using a lean NOx trap to store NOx emissions during fuel lean operations and then purging the stored NOx during fuel rich, higher temperature engine operating conditions with conventional three-way catalysis to nitrogen and water. Diesel particulate filters (DPF) trap soot and particulate matter in diesel applications, and the trapped material is periodically purged in high temperature regeneration events.
Urea injection in a powertrain can be problematic. Urea storage and replenishment can be difficult to maintain. Urea is prone to freeze under normally varying climatic conditions in common regions.